Ectonucleotide Pyrophophatase/Phosphodiesterase (ENPP) family members include seven isoforms, ENPP1-7, which are type II transmembrane glycoproteins or ectoenzymes. Mass spectrometry and proteomics analysis from more than 370 protein targets led to the identification of an extracellular protein ENPP1 as one of the top hit which exhibited high hydrolytic activity. ATP is an identified substrate of ENPP1, which is hydrolyzed to AMP and PPi. CD73 converts AMP to adenosine and inorganic phosphate (Pi). The kinetic experimental data indicates that the ENPP1 is capable of hydrolyzing ATP. These ectonucleotide enzymes are involved in the hydrolysis of pyrophosphate (PPi) and phosphodiester bonds in extracellular nucleotides; such as triphosphates, oligonucleotides and that generates nucleoside 5′-monophosphates. One of the key isoforms, ENPP1 (Plasma cell membrane glycoprotein-1, PC-1), is involved in a number of physiological processes, such as development, formation and trafficking, as well as in pathophysiological conditions. Aberrant ENPP1 expression has been detected in breast cancers relative to normal mammary epithelium, an evidence of its potential in the development of bone metastasis (occurs in approximately 80% cases), Hodgkin's lymphoma, hepatocellular carcinoma, follicular lymphoma, glioblastoma and in other malignant tumor tissues.
Recent reports suggest that the cyclic dinucleotides (CDNs), a substrate for ENPP1, stimulate innate immunity via STING-dependent activation of interferon genes. ENPP1 inhibition of STING pathway activation is critical for tumor control, similar to that of checkpoint inhibitors such as anti PD-1 or PD-L1 which are promising immunotherapeutics for various cancers. In addition, mutations in ENPP1 were associated with several disorders including infantile arterial calcification (generalized arterial calcification of infancy or GACI), ossification of the posterior longitudinal ligament of the spine and insulin signaling and resistance. ENPP1 expression is high in bone and cartilage and is implicated in lung and kidney fibrosis. A correlation was also found between expression of ENPP1 and the grade of astrocytic tumor. Another study reported that ENPP1 was required to maintain the undifferentiated and proliferative state of glioblastoma stem-like cells. Therefore, ENPP1 is an attractive druggable target for the development of novel anticancer, cardiovascular, diabetes, obesity and anti-fibrotic therapeutics.
Importance of ENPP1 activity was further investigated from both direct binding assay and in vitro cellular efficacy on MDA-MB231 cells. The siRNA-based knock down of ENPP1 significantly reduced its catalytic activity both in cell specific and in vivo experiments. These experiments demonstrated that the ENPP1 activity was abolished on treatment with siRNA. This further supports the validity of this target in certain diseases. It has been shown recently that the bisphosphothionate analog of endogenous cGAMP is resistant to hydrolysis by ENPP1 phosphodiesterase, and particularly the cyclic dinucleotides (CDNs) are more potent at inducing IFN-β secretion in human THP1 cells by a mechanism of inhibiting the ENPP1 activity and simultaneous STING activation responses.
There is ample evidence that ENPP1 expression is prominent in human primary breast tumors relative to normal mammary epithelium, with highest levels observed in breast-bone metastasis. These data not only support a potential role for ENPP1 in breast-bone metastasis, but also support as a potential prognostic marker for breast cancer. These results from target validation experiments clearly support the pharmacological role of ENPP1 for the development of novel immunotherapeutics for cancers.
Furthermore, ENPP1 activity has also been implicated in diseases caused by bacteria and/or viruses, and therefore modulators of ENPP1 can be used to treat bacterial and/or viral diseases and conditions.